How Fish use Colour in the Ocean
Written by Roy Kittrell 3 Nov 2023.
For this month’s article, I am going to look at some of the interesting and sometimes even surprising ways that fish use colour underwater.
1. Camouflage
The first and perhaps most obvious use of colour for fish is to camouflage themselves against the seafloor to avoid predators! When I was on the Flow Dive Center’s trip to Bali this year, I saw a fantastic example of this when I was swimming over a flatfish, a Pacific Peacock Flounder (Bothus mancus) to be exact. What was so stunning to me, was that not only was this flounder extremely well camouflaged, but that it changed its colours instantly once it settled on the sea floor! The pictures below are taken merely seconds apart from each other.
And to answer your question, yes, fish can actively change the colour of their skin, just like octopuses or chameleons! They even do this in much the same way. Fish have two types of cells in their skin that give colour, Chromatophores and Iridophores. Iridophores are highly reflective cells containing the crystal guanine, and alter the reflectivity of the fish. In a lot of open water pelagic fish, these cells are what make them look silver. Chromatophores are a type of cell that contains a single colour, and enlarging various colours of these cells the fish can alter its appearance, as we can see above, they can do this quite rapidly.
Sometimes though camouflage doesn’t mean what you would normally think. Whereas the flounder’s colouration becomes duller against the sea floor backdrop other fish actually become more colourful to blend in with their surroundings. The painted frogfish (Antennarius pictus), seen below on another Flow trip to the Lembeh Straits, is an extremely vivid orange to make itself look more like one of the many colourful sponges we see in the reefs. Another stunning fish, the Harlequin Ghost Pipefish (Solenostomus paradoxus), camouflages itself by mimicking the vibrant colours of the feather stars that it likes to live in and around
2. Warning Colours
If some fish use colour to camouflage their bodies so that they blend in, others might use their colours to stand out!
Such is the case for the Mandarin Fish (Synchiropus splendidus), a dazzlingly coloured variety of dragonet that we saw on the Flow Dive trip to Lembeh Straits. The colours here are a warning to predators that they are poisonous or distasteful, a kind of display that is known as an aposematic display in biology. In mandarin fish, this is a true aposematic display because the mucus on their skin is highly distasteful to predators. Mandarins are also famous for their intricate mating ritual, which we were lucky enough to witness for ourselves on a night dive there.
3. False Warning Colours
Some fish are brightly coloured as an honest warning to predators, and then there are some similarly brightly coloured fish in the ocean who are lying, and only pretending to be poisonous in order to fool predators.
Such is the case for many fish when they are in their juvenile stages, for example this perfectly edible juvenile Kuiter’s dragonet (Dactylopus kuiteri), which is mimicking the warning colours of a nudibranch, something that actually is poisonous and/or distasteful to predators. This bluffing colouration is a phenomenon known as Batesian mimicry, and many fish do this as juveniles, only taking on their normal adult colours when they grow up.
4. Communication
Another way that fish use colour is for communication. The Bigeye Trevally (Caranx sexfasciatus), often seen in beautiful big schools around Sipadan Island, is a great example of this. Normally, the fish are silver in colour, due to the iridescence of the Iridophores present in a lot of pelagic fish as I mentioned earlier, but when the males are ready to mate, they signal to the females by turning a jet black, and pairing up, as seen in the picture below.
Fish don’t just use colour to signal to other fish, sometimes they even directly communicate with other species! One example of this was seen in the Blue Planet documentary series, where a grouper fish helps an octopus hunt for smaller fish by looking into the coral with its excellent eyesight, and flashing different colours to tell the octopus where the fish is. The octopus, which can reach into lots of smaller nooks and crannies in the coral, flushes the fish out and both the grouper and octopus get to feed.
What is quite remarkable about this, is that the grouper and the octopus are from entirely different phylums (Animalia, and Mollusca respectively). The clip of this amazing behaviour can be seen on YouTube.
Flow Dive Center just released its 2024 dive trips! Make sure you head on over and book a trip to catch these amazing creatures in action in real life!
– Roy Kittrell is an avid naturalist and underwater photographer, his work can be found on instagram @roythedivebro